I do not know what the intention was for sequences and named placeholders. Thinking of named tuples made me think it ok. The code might have a hint.

Is sqlite3 code maintained here or elsewhere? The current docstring is just 'Executes a SQL statement.', and help gives no signature. .executemany is similar. I suspect the whole module needs better docstrings.

You are correct about dicts and extra key:value pairs. I tried a UserDict with an extra pair and when I got

sqlite3.ProgrammingError: Incorrect number of bindings supplied. The current statement uses 2, and there are 3 supplied

I had not yet realized that they were treated as sequences, not dicts. So replace "Len(seq_dict)" with "The length of sequences". This solves Chris's point also.